1. Field of the Invention
The present invention relates to a method for transmitting and receiving a signal by efficiently processing a new data indicator (NDI) received and stored in a user equipment (UE) during a random access procedure of a mobile communication system, and a user equipment for the method.
2. Discussion of the Related Art
As an example of a mobile communication system to which the present invention can be applied, a third generation partnership project long term evolution (3GPP LTE) communication system will be described in brief.
FIG. 1 is a diagram illustrating a network structure of an E-UMTS (Evolved-Universal Mobile Telecommunications System) which is a mobile communication system. An E-UMTS is a system evolving from the conventional universal mobile telecommunication system (UMTS) and its basic standardization is currently handled by the 3GPP. Generally, The E-UMTS may be called a long term evolution (LTE) system.
The E-UMTS network may largely be classified into a UMTS terrestrial radio access network (E-UTRAN) 101 and a core network (CN) 102. The E-UTRAN 101 includes a user equipment (UE) 103, a base station (eNode-B or eNB) 104, and an access gateway (AG) which is located at an end of the network and is connected to an external network. The AG 105 may be classified into a part for handling user traffic and a part for handling control traffic. At this time, an AG for handling new user traffic may communicate with another AG for handling control traffic via a new interface.
At least one cell exists in one eNB. An interface for transmitting user traffic or control traffic may be located between eNBs. The core network (CN) 102 can include a node for user registration of other user equipment (UE) 103 and the access gateway 105. An interface for discriminating between the E-UTRAN 101 and the CN 102 may also be used.
Layers of a radio interface protocol between a UE and a network can be classified into a first layer L1, a second layer L2 and a third layer L3 based on three lower layers of an OSI (open system interconnection) standard model widely known in communication systems. A physical layer belonging to the first layer L1 provides an information transfer service using a physical channel. A radio resource control (hereinafter, abbreviated as ‘RRC’) layer located at the third layer plays a role in controlling radio resources between the UE and the network. For this, the RRC layer enables RRC messages to be exchanged between the UE and the network. The RRC layer may distributively be located at network nodes including the eNode B 104, the AG 105 and the like, or may independently be located at either the eNode B 104 or the AG 105.
FIG. 2 and FIG. 3 are diagrams illustrating a structure of a radio interface protocol between a user equipment and UTRAN based on the 3GPP radio access network standard.
The radio interface protocol of FIG. 2 and FIG. 3 is horizontally divided into a physical layer PHY, a data link layer and a network layer, and is vertically divided into a user plane for transmitting data information and a control plane for transmitting control signaling. In detail, FIG. 2 illustrates layers of the radio protocol control plane and FIG. 3 illustrates the layers of the radio protocol user plane. The protocol layers of FIG. 2 and FIG. 3 may be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on the three lower layers of an open system interconnection (OSI) standard model which is well-known in the art of communication systems.
Hereinafter, each layer of the radio protocol control plane of FIG. 2 and the radio protocol user plane of FIG. 3 will be described.
The physical layer PHY, which is the first layer, provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected with a medium access control (MAC) layer located at a higher level through a transport channel, and data between the MAC layer and the physical layer is transferred via the transport channel. At this time, the transport channel is divided into a dedicated transport channel and a common transport channel depending on channel sharing. Between different physical layers, namely, between physical layers of a transmitter and a receiver, data is transferred via the physical channel using radio resources.
Several layers exist in the second layer. First of all, a medium access control (MAC) layer of the second layer serves to map various logical channels with various transport channels. Also, the MAC layer performs multiplexing for mapping several logical channels with one transport channel. The MAC layer is connected with an RLC layer corresponding to its upper layer through the logical channel. The logical channel is divided into a control channel and a traffic channel depending on types of transmitted information, wherein the control channel transmits information of the control plane and the traffic channel transmits information of the user plane.
The RLC layer of the second layer serves to perform segmentation and concatenation of data received from its upper layer to control a size of the data so that the lower layer transmits the data to a radio-communication interval. Also, the RLC layer of the second layer provides three action modes, i.e., a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM) to ensure various quality of services (QoS) required by each radio bearer (RB). In particular, the AM RLC layer performs a retransmission function through automatic repeat and request (ARQ) for reliable data transmission.
In order to effectively transmit data using IP packets (e.g., IPv4 or IPv6) within a radio-communication interval having a narrow bandwidth, a PDCP (packet data convergence protocol) layer of the second layer (L2) performs header compression to reduce the size of IP packet header having relatively great size and unnecessary control information. The header compression is to increase transmission efficiency of the radio-communication period by allowing a packet header of data to transmit necessary information only. Also, in the LTE system, the PDCP layer performs a security function. The security function includes a ciphering function preventing the third party from performing data monitoring and an integrity protection function preventing the third party from performing data manipulation.
A radio resource control (hereinafter, abbreviated as ‘RRC’) layer located on the uppermost of the third layer is defined in the control plane only and is associated with configuration, re-configuration and release of radio bearers (hereinafter, abbreviated as ‘RBs’) to be in charge of controlling the logical, transport and physical channels. In this case, the RB means a logical path provided by the first and second layers of the radio protocol for the data transfer between the user equipment and the UTRAN. Generally, establishing RB means a procedure of defining features of a radio protocol layer and channel required for a specific service and establishing detailed parameters and action methods of the radio protocol layer and the channel. The RB is divided into a signaling RB (SRB) and a data RB (DRB). The SRB is used as a path for transmitting RRC message in a control plane (C-plane), and the DRB is used as a path for transmitting user data in a user plane (U-plane).
As downlink transport channels carrying data from the network to the user equipments, there are provided a broadcast channel (BCH) carrying system information and a downlink shared channel (SCH) carrying user traffic or control messages. The traffic or control messages of a downlink multicast or broadcast service may be transmitted via the downlink SCH or an additional downlink multicast channel (MCH). Meanwhile, as uplink transport channels carrying data from the user equipments to the network, there are provided a random access channel (RACH) carrying an initial control message and an uplink shared channel (UL-SCH) carrying user traffic or control messages.
As downlink physical channels carrying information transferred to a downlink transport channel to a radio interval between a network and a user equipment, there are provided a physical broadcast channel (PBCH) transmitting information of the BCH, a physical multicast channel (PMCH) transmitting information of the MCH, a physical downlink shared channel (PDSCH) transmitting information of the PCH and the downlink SCH, and a physical downlink control channel (PDCCH) (or DL L1/L2 control channel) transmitting information control information provided by the first layer and the second layer, such as downlink or uplink radio resource assignment information (DL/UL scheduling grant). Meanwhile, as uplink physical channels transmitting information transferred to an uplink transport channel to a radio interval between a network and a user equipment, there are provided a physical uplink shared channel (PUSCH) transmitting information of the uplink SCH, a physical random access channel (PRACH) transmitting RACH information, and a physical uplink control channel transmitting control information provided by the first layer and the second layer, such as HARQ ACK or NACK, scheduling request (SR), and channel quality indicator (CQI) report.
Hereinafter, a random access procedure provided by the LTE system will be described based on the aforementioned description.
First of all, the user equipment performs a random access procedure in case of the following cases:                when the user equipment performs initial access as the user equipment is not RRC connected with the base station;        when the user equipment first accesses a target cell during a handover procedure;        when a random access procedure is requested by a command of the base station;        when data to be transmitted to an uplink occurs in a state that time synchronization of an uplink is not appropriate or a designated radio resource is not assigned; and        when the user equipment performs a recovery procedure during a radio link failure or handover failure.        
The LTE system provides both a contention based random access procedure and a non-contention based random access procedure during a procedure of selecting a random access preamble. In the contention based random access procedure, the user equipment randomly selects one preamble from a specific set and uses the selected preamble. In the non-contention based random access procedure, the base station uses a random access preamble assigned to a specific user equipment. However, the non-contention based random access procedure can be used only in the aforementioned handover procedure or as requested by a command of the base station.
Meanwhile, the procedure of performing random access in the user equipment with a specific base station includes the steps of (1) transmitting a random access preamble from the user equipment to the base station (“first message (message 1)” transmitting step), (2) receiving a random access response message from the base station in response to the transmitted random access preamble (“second message (message 2)” receiving step), (3) transmitting an uplink message using information received in the random access response message (“third message (message 3)” transmitting step), and (4) receiving a message corresponding to the uplink message from the base station (“fourth message (message 4)” receiving step).
In a next generation mobile communication system including the LTE system, a hybrid automatic repeat request (HARQ) method is used as a method for efficiently performing uplink and/or downlink signal transmission including the aforementioned random access procedure. The HARQ method is a combined type of an ARQ scheme and a forward error correction (FEC) scheme. According to the ARQ scheme, a transmitter retransmits a signal of a receiver, which is failed in reception, by feeding ACK/NACK signal back depending on whether a received signal is successfully decoded. The FEC scheme is to acquire coding gain and/or SINR gain by combining the failed signal with the retransmitted signal to correct an error of the received signal. In order to perform the aforementioned HARQ scheme, the base station can transmit a new data indicator (NDI) through the PDCCH, wherein the NDI is to indicate the user equipment whether the user equipment performs new data transmission or data retransmission to the uplink. Likewise, the base station can transmit an NDI through the PDCCH, wherein the NDI is to indicate the user equipment whether the base station performs new data transmission or data retransmission to the downlink.
Generally, an NDI field is a 1-bit field, and is toggled in the order of 0->1->0->1-> . . . whenever new data is transmitted. In case of retransmission, the NDI field has the same value as that of initial transmission. Namely, the user equipment compares a previously transmitted value with the NDI field to identify whether data retransmission is performed.
Meanwhile, a plurality of HARQ processes are operated independently within the user equipment. Since independent data transmission is performed for each of the HARQ processes, NDI is established for each of the HARQ processes. In this case, a specific HARQ process may be involved in general uplink/downlink data transmission and reception including the aforementioned random access procedure in accordance with data. More detailed studies of an action of NDI in determining whether the user equipment determines retransmission during uplink/downlink data transmission and reception after a random access procedure will be required, wherein the NDI is received to correspond to a specific HARQ process during the random access procedure.